1. Field of the Invention
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to direct current electrical switching apparatus, such as, for example, direct current circuit breakers. The disclosed concept further pertains to a direct current arc chamber including a magnetic member having a lower surface disposed below the fixed contact.
2. Background Information
Electrical switching apparatus employing separable contacts exposed to air can be structured to open a power circuit carrying appreciable current. These electrical switching apparatus, such as, for instance, circuit breakers, typically experience arcing as the contacts separate and commonly incorporate arc chambers, such as arc chamber assemblies, to help extinguish the arc. Such arc chamber assemblies typically comprise a plurality of electrically conductive plates held in spaced relation around the separable contacts by an electrically insulative housing. The arc transfers to the arc plates where it is stretched and cooled until extinguished.
Known molded case circuit breakers (MCCBs) are not specifically designed for use in direct current (DC) applications. When known alternating current (AC) MCCBs are sought to be applied in DC applications, multiple poles are electrically connected in series to achieve the required interruption or switching performance based upon the desired system DC voltage and system DC current.
One of the challenges in DC current interruption/switching, especially at a relatively low DC current, is to drive the arc into the arc interruption chamber. Known DC electrical switching apparatus employ permanent magnets to drive the arc into arc splitting plates. Known problems associated with such permanent magnets in known DC electrical switching apparatus include unidirectional operation of the DC electrical switching apparatus, and two separate arc chambers each including a plurality of arc plates and a set of contacts must be employed to provide bi-directional operation. These problems make it very difficult to implement a permanent magnet design for a typical DC single-pole MCCB without a significant increase in size and cost.
An electrical switching apparatus with a permanent magnet arrangement and single break operation may be used to achieve bi-directional DC switching and interruption. For example, two permanent magnet plates employed along both sides of a single arc chamber include a single set of a plurality of arc plates and a permanent magnet or ferromagnetic center barrier to provide a dual arc chamber structure. The resulting magnetic field drives the arc into one side of the dual arc chamber structure and splits the arc accordingly depending upon the direction of the DC current. Such a single direct current arc chamber includes a ferromagnetic base having a first end and an opposite second end; a first ferromagnetic side member disposed from the first end of the ferromagnetic base; a second ferromagnetic side member disposed from the opposite second end of the ferromagnetic base; a third ferromagnetic member disposed from the ferromagnetic base intermediate the first and second ferromagnetic side members; a first permanent magnet having a first magnetic polarity disposed on the first ferromagnetic side member and facing the third ferromagnetic member; and a second permanent magnet having the first magnetic polarity disposed on the second ferromagnetic side member and facing the third ferromagnetic member.
Such an arc chamber can still be improved. That is, as the arc created during the separation of the contacts moves from the contacts to the arc plates, the arc may impinge upon the housing for the magnetic members disposed on either side of the contacts. Further, the arc may experience a fringing effect that can impede the progress of the arc. That is, if the lower edges of the permanent magnets are at or near the fixed contact surface level, the magnetic field generated by the permanent magnets close to the fixed contact region is either significantly reduced or reverses its direction. This reversed magnetic field will drive the arc in an opposite direction away from the arc interruption chamber. The reduction or reversion of the magnetic field near the edge of a permanent magnet is called the fringing effect.
There is, therefore, a need for an improved arc chamber configured to control the path of travel of the arc. There is a further need for such an arc chamber to be compatible with existing circuit breaker housings.